I would like to add the class incision with regex and preg_replace
echo preg_replace("/<li\>\s*<p\>[a-z]\)\s/", "/<li class=\"inciso\"\>\s*<p\>[a-z]\)\s/", $documento);
This is the template for the lines in my document:
<li>
<p>a) longo texto</p>
</li>
I do not think you understood how preg_replace work because you passed as second parameter a regular expression, when should use the text that should replace what was found.
In addition you are escaping (with backslash) the > character that has no special function in the regular expression.
preg_replace('/<li>(\s*<p>[a-z]\)\s)/', '<li class="inciso">$1', $documento);
Explanation of this regular expression can be found in this answer
Using Regex with HTML not a good idea .
Instead you can use DOM :
<?php
$dom = new DomDocument;
$dom->loadHTML("<li><p>a) longo texto</p></li>");
$lis = $dom->getElementsByTagName('li');
foreach ($lis as $li) {
$li->setAttribute('class', 'inciso');
}